Liquid crystal materials are useful for electronic displays because light traveling through a layer of liquid crystal (LC) material is affected by the anisotropic or birefringent value (.DELTA.N) of the material, which in turn can be controlled by the application of a voltage across the liquid crystal material. Liquid crystal displays are desirable because the transmission or reflection of light from an external source, including ambient light and backlighting schemes, can be controlled with much less power than was required for the illuminance materials used in other previous displays. Liquid crystal displays are now commonly used in such applications as digital watches, calculators, portable computers, avionic cockpit displays, and many other types of electronic devices which utilize the liquid crystal display advantages of long life and operation with low voltage and power consumption.
The information in many liquid crystal displays is presented in the form of a matrix array of rows and columns of numerals or characters, which are generated by a number of segmented electrodes arranged in such a matrix pattern. The segments are connected by individual leads to driving electronics, which apply a voltage to the appropriate combination of segments to thereby display the desired data and information by controlling the light transmitted through the liquid crystal material. Graphic information in, for example, avionic cockpit applications or television displays may be achieved by a matrix of pixels which are connected by an X-Y sequential addressing scheme between two sets of perpendicular conductor lines (i.e. row and column lines). More advanced addressing schemes use arrays of thin film transistors or diodes which act as switches to control the drive voltage at the individual pixels. These schemes are applied predominantly to twisted nematic liquid crystal displays, but are also finding use in high performance versions of super twisted liquid crystal displays.
Contrast is one of the most important attributes determining the quality of both normally white (NW) and normally black (NB) liquid crystal displays. In normally black (NB) LCDs, the primary factor limiting the contrast achievable in these liquid crystal displays is the amount of light which leaks through the display in the darkened or OFF state. In the NW mode, the primary factor limiting the contrast is the amount of light which leaks through the display in the darkened ON state. This problem is compounded in a bright environment, such as sunlight, where there is a considerable amount of reflected and scattered ambient light. In color liquid crystal displays, light leakage causes severe color shifts for both saturated and gray scale colors. These limitations are particularly important for avionics applications, where copilot viewing of the pilot's displays is important.
In addition, the legibility of the image generated by both normally black (NB) and normally white (NW) liquid crystal devices depends on the viewing angle, especially in a matrix addressed device with a large number of scanning electrodes. Absent a retardation film, the contrast ratio in a typical NB or NW liquid crystal display is usually at a maximum only within a narrow viewing (or observing) angle centered about normal incidence (0.degree. horizontal viewing angle and 0.degree. vertical viewing angle) and drops off as the angle of view is increased.
It would be a significant improvement in the art to provide a liquid crystal display capable of presenting a high quality, high contrast image over a wide field of view.
Several types of liquid crystal pixels or cells are in widespread use in flat panel displays. Active matrix addressing allows such displays to present a full color image with high resolution. When viewed directly at a normal or on-axis viewing angle N (0.degree. vertical viewing angle and 0.degree. horizontal viewing angle), a liquid crystal display provides a generally high quality output especially when the cell gap "d" is matched to the first transmission minimum, but the image degrades and exhibits poor contrast at increased viewing angles. This occurs because liquid crystal cells operate by virtue of the anisotropic or birefringent effect exhibited by a liquid crystal layer which includes a large number of anisotropic liquid crystal molecules. Such a material will be positively uniaxially birefringent (i.e., the extraordinary refractive index is larger than the ordinary refractive index) with an extraordinary refractive index associated with the alignment of the long molecular axes. The phase retardation effect such a liquid crystal material has on light passing through it inherently varies or increases with the inclination angle of light, leading to a lower quality image at larger viewing angles (see, e.g. Penz, Viewing Characteristics of the Twisted Nematic Display, Proceeding of the S.I.D., Vol. 19, p. 43 (1978); Grinberg, et al., Transmission Characteristics of a Twisted Nematic Liquid Crystal Layer, Journal of the Optical Society of America, Vol. 66, p. 1003 (1976)). By introducing an optical compensating element (or retarder) into the liquid crystal pixel or cell, however, it is possible to correct for the unwanted angular effects and thereby maintain higher contrast at both normal and larger viewing angles than otherwise possible.
The type and orientation of optical compensation or retardation required depends upon the type of display, normally black or normally white, which is used.
In a normally black (NB) twisted nematic display, the twisted nematic liquid crystal material is placed between polarizers whose transmission axes are parallel to one another. Such NB displays may be either front X-buffed or rear X-buffed. The first and second LC buffing zones are preferably perpendicular to one another thereby necessitating one of the buffs being perpendicular relative to the polarizer axes. If the first buff zone is perpendicular to the first polarizer transmission axis then the display is rear "X-buffed." Otherwise, it is front "X-buffed."
In the unenergized or OFF state (no voltage above the threshold voltage V.sub.th is applied across the liquid crystal material), normally incident light from the backlight is first polarized by the first polarizer and in passing through the pixel or cell has its polarization direction rotated by the twist angle of the liquid crystal material dictated by the buffing zones. This effect is known as the waveguiding or twisting effect. The twist angle is set, for example, to be about 90.degree. so that the light is blocked or absorbed by the second or output polarizer. When a voltage is applied via electrodes across the normally black pixel, the liquid crystal molecules are forced to more nearly align with the electric field, eliminating the twisted nematic symmetry of the LC material. In this orientation, the optical molecular axes of the liquid crystal layer molecules are perpendicular to the cell walls. The liquid crystal layer then appears isotropic to normally incident light, eliminating the waveguiding effect such that the light polarization state is unchanged by propagation through the liquid crystal layer so that light can pass through the second or output polarizer. Patterns can be written in the NB display by selectively applying a variable voltage to the portions of the display which are to appear illuminated.
When viewed in the OFF state at both normal N and other viewing angles, however, the dark unenergized areas of a normally black display will appear colored because of angle dependent retardation effects for light passing through the liquid crystal layer at such angles. Contrast can be restored by using a compensating or retarding element which has an optical symmetry similar to that of the twisted liquid crystal layer but which reverses its effect. One method is to follow the active liquid crystal layer with another twist liquid crystal cell of reverse helicity. Another, in an NB cell, is to use one or more plate retarder compensators each having a constant birefringent value throughout the pixel. These compensation methods work because the compensation or retardation element shares an optical symmetry with the twisted nematic liquid crystal material in that both are preferably uniaxial birefringent materials having extraordinary axes orthongonal to the normal light propagation direction. These approaches to compensation have been widely utilized because of the ready availability of materials with the required optical symmetry. Reverse twist cells employ liquid crystals while retardation plates are readily manufactured by the stretching of the polymers such as polyvinyl alcohol (PVA). Regarding the reverse twist cell compensation technique discussed above, this requires the insertion of a second liquid crystal cell into the optical path, adding significant cost, weight and bulk to the display.
Despite the effectiveness of these compensation techniques, there are drawbacks to these approaches associated with the normally black operational mode. The appearance of a normally black display is very sensitive to cell gap "d." Consequently, in order to maintain a uniform dark appearance in the OFF state, it is necessary to match the thickness "d" of the liquid crystal layer to the first transmission minimum of each particular wavelength or color used in the pixel. This is illustrated in prior art FIG. 1 (see, for example, U.S. Pat. No. 4,632,514) which shows a multi-colored pixel for a liquid crystal display including a blue subpixel, a green subpixel and a red subpixel, wherein the thickness or cell gap "d" of the liquid crystal layer 15 varies according to the color or wavelength of each subpixel so as to match "d" to the first transmission minimum of each color. Such multi-gap displays are very difficult and expensive to manufacture.
Therefore, it would be highly desireable to provide a liquid crystal display including red, green, and blue subpixels as shown in FIG. 1, which has good color contrast ratios and compensates for the different color wavelengths but does not require varying the thickness "d" of the liquid crystal layer according to each color so as to selectively match "d" to the first transmission minimum of the wavelength of each subpixel color (red, green, blue).
Turning now to NW LCD cells, in a normally white liquid crystal display configuration, a twisted nematic cell preferably having a twist angle of about 90.degree. is placed between polarizers which have crossed or perpendicular transmission axes, such that the transmission axis of each polarizer is parallel or perpendicular to the buffing direction of orientation of the liquid crystal molecules in the interface region of the liquid crystal material adjacent to each polarizer. In other words, NW cells can be either P-buffed wherein both polarizer axes are parallel to their respective adjacent buffing zones, or X-buffed wherein both polarizer axes are perpendicular to their respective buffing zones. This orientation of the polarizers reverses the sense of light and dark from that of the normally black display discussed above. The OFF or unenergized (no applied voltage above V.sub.th across the liquid crystal material) areas appear light in a normally white display, while those which are energized appeared dark. The problem of ostensibly dark areas appearing light or colored when viewed at large angles still occurs, however, the reason for it is different. Either positive or negative birefringent retarders may be used to correct the NW displays, depending upon their orientation. In the NW energized darkened areas, the liquid crystal molecules tend to align with the applied electric field. If this alignment were perfect, all of the liquid crystal molecules in the cell would have their long axes normal to the glass substrate or cell wall. In the energized state, the normally white display appears isotropic to normally incident light, which is blocked by the crossed polarizers, thus, resulting in a darkened pixel or subpixel.
The loss of contrast with increased viewing angles in NW pixels or displays occurs primarily because the homeotropic liquid crystal layer does not appear isotropic to off axis or off normal light. Light directed at off normal angles propagates in two modes due to the anisotropy or birefringence (.DELTA.N) of the liquid crystal layer, with a phase delay between these modes which increases with the incident angle of light. This phase dependence on the incident angle introduces an ellipticity to the polarization state which is then incompletely extinguished by the second polarizer, giving rise to light leakage. Because of the NW symmetry the birefringence has no azimuthal dependence.
Accordingly, what is often needed in normally white liquid crystal cells is an optical compensating or retarding element which would introduce a phase delay opposite in sign to that caused by the liquid crystal layer, thereby restoring the original polarization state, allowing the light to be blocked by the output polarizer. Optical compensating elements or retarders with such NW symmetry and often negative birefringence are known in the art and are disclosed, for example, in U.S. Pat. Nos. 5,196,953, 5,138,474, and 5,071,997, the disclosures of which are hereby incorporated herein by reference. It is known that the polyimides and co-polyimides disclosed by aforesaid U.S. Pat. No. 5,071,997 can be used as retarding elements in NW liquid crystal displays and are said to be custom tailorable to the desired negative birefringent values without the use of stretching.
Turning once again to FIG. 1, there is illustrated a prior art normally black liquid crystal display pixel including three colored subpixels. Optical radiation from a radiation source is applied to the liquid crystal display pixel of FIG. 1. The applied optical radiation schematically illustrated as 2A, 2B, and 2C, is typically from a single source, but is shown in FIG. 1 in terms of the component or subpixel units of the display pixel. The optical radiation first passes through first linear polarizer 14. The optical radiation is then applied to the liquid crystal cell 10. The liquid crystal cell 10 is bounded by two transparent glass substrates 11 and 12. On the interior surface of the glass substrate 12 are transparent conducting regions 18A, 18B, and 18C. These conducting regions are electrodes for applying an electric field to the liquid crystal layer 15 of each subpixel color component unit of the display pixel. The blue subpixel has a blue optical filter 16A; the green subpixel has a green optical filter 16B; and the red subpixel has a red optical filter 16C. These optical filters are coupled to the second glass substrate 11.
Deposited on the optical filters is a transparent conducting material 17 which acts as the second electrode for each subpixel of the liquid crystal pixel. A power supply 4 is provided to illustrate that a potential can be applied to the liquid crystal material 15 which occupies the region between the electrodes 18A, 18B, and 18C and the second electrode 17. As will be clear to those familiar with liquid crystal displays, the power supply 4 is typically replaced by addressing circuitry for applying a predetermined voltage to each of the subpixel electrodes. In this manner, an image can be displayed to a viewer (or observer).
The optical radiation 19A, having been linearly polarized by the first polarizer 14, is rotated about 90.degree. during transmission through the liquid crystal material 15 between the first electrode 18A and second electrode 17. The linearly polarized optical radiation 19B and 19C are similarly rotated about 90.degree. in the different color subpixels of the pixel. The optical radiation, after transmission through the liquid crystal material 15 passes through one of the color filters 16A, 16B, and 16C. The optical color filters select the color components for their respective subpixels to be transmitted by the color subpixels of the liquid crystal display. However, the different wavelengths (e.g. red, green, and blue) are affected to different extents by the birefringence of the LC material thereby necessitating the multi-gap configuration shown in FIG. 1 and creating different relative color leakages at different viewing angles.
After transmission through the liquid crystal material, the optical radiation is transmitted through the retardation plates 21 and 22. The off axis transmission, as discussed above, becomes increasingly elliptically polarized with increased angle, a result of the birefringence of the liquid crystal material. The result of this elliptical polarization is a reduction of radiation contrast as a function of angle about the normal axis N after transmission of the radiation through the second linear polarizing plate 13. In order to compensate for the angular dependent reduction in contrast, retardation plates 21 and 22 of constant retardation value are interposed between the substrate 11 and the polarizer 13 as shown in FIG. 1. The presence of the retardation plates 21 and 22 results in a decrease in the elliptical polarization of the radiation applied to the linear polarizing plate 13. Consequently, the angle dependent variation in contrast ratio of the radiation transmitted through the second linear polarizing plate 13 is improved.
Furthermore, as shown in FIG. 1, the multi-gap aspect of this prior art pixel requires the thickness "d" of each subpixel being selected so as to match the optical path difference (d.multidot..DELTA.N).div..lambda. of the liquid crystal cell 15 to the first transmission minimum of each respective color of the three subpixels. Accordingly, because each color (red, green, and blue) has a different wavelength and the birefringent value AN of the liquid crystal material remains constant, the thickness "d" of each subpixel must be adjusted accordingly so as to compensate for the different wavelengths of each color and the cell is thereby optimized for the normal viewing angle N. The normal viewing angle herein is shown by reference element "N" and means about a 0.degree. horizontal and vertical viewing angle.
Reference next is to prior art FIG. 2, which illustrates schematically how the light travels in the LCD of FIG. 1. As illustrated, the incoming radiation 2 is first transmitted through first linear polarizer 14. The next optically oriented region through which the optical radiation passes is the first orientation film or surface 18S of the conducting plates with which the liquid crystal material 15 is in contact. The surface 18S has an orientation or buffing parallel to the first linear polarizer 14. Ignoring for purposes of this discussion the controllable orientation of the actual liquid crystal material, the next optically oriented region through which the optical radiation is transmitted is the second orientation film or buffed surface 17S of the second conducting electrode 17, the second surface to which the liquid crystal material 15 is exposed. The surface 17S is oriented or buffed in a direction perpendicular to the surface 18S to which the liquid crystal is exposed thereby creating about a 90.degree. twist in the LC material. The retardation plate 21, having a constant anisotropic or birefringent value (.DELTA.N) as well as a constant retardation value throughout all three subpixels, has an optical axis oriented parallel to the orientation of the surface 17S, while retardation plate 22, also having a constant birefringent value throughout all three subpixels, has its optical axis oriented at right angles to the axis orientation of retardation plate 21. The retardation value of a retardation plate or film is determined by the formula "d.multidot..DELTA.N," wherein "d" is the thickness of the plate or film and ".DELTA.N" is the birefringent value of the plate. Finally, when the pixel is in the ON or energized state, the optical radiation is transmitted through the second linear polarizer 13 which is oriented parallel to retardation plate 22 and linear polarizer 14.
Referring next to FIG. 3, which is a graph illustrating the different transmission minimums of red, green, and blue wavelengths in a normally black liquid crystal cell, the percent optical transmission through a liquid crystal cell in the absence of an applied electric field as a function of distance "d" in the liquid crystal material through which the optical radiation travels is illustrated for the typical color components. For substantially no transmission of optical radiation in the OFF state, the transmission minimum for blue radiation occurs at approximately a thickness of the liquid crystal material "d (blue)" the transmission minimum for green radiation occurs at a thickness of liquid crystal material of "d (green)" which is greater than "d (blue)", and the transmission minimum for red radiation occurs at a thickness of liquid crystal material of "d (red)" which is greater than "d (green)". This difference in the minimum of the transmitted radiation of each color is, as discussed above, the reason that the cell thickness "d" of each subpixel is different in the multi-gap configuration of FIG. 1.
A drawback of the prior art liquid crystal display discussed above such as has been illustrated and discussed with reference to FIGS. 1-2, is that the thicknesses "d" of the LC material must be finely adjusted to match the first transmission minimum of each color, and furthermore, the retardation film(s) 21 and 22 have a single retardation value applicable to all of the colored subpixels and do not take into consideration the different wavelengths. Because of the constant retardation values of the retardation films for all of the subpixels, the result is that at different viewing angles, there are different viewing leakages for the different colors (red, green, and blue). The NB pixel shown in FIG. 1, for example, when viewed in the OFF state at a normal viewing angle N experiences a blue leakage, because the single constant value of the retardation plates or films is substantially matched to the green wavelength at a normal viewing angle. However, at increased horizontal viewing angles, the pixel of FIG. 1 experiences green and red leakage while properly transmitting the blue color.
In the case of obliquely angled light traveling through the pixel shown in FIG. 1, the normal component or vector is twisted about 90.degree. by the liquid crystal material but the horizontal component is twisted to another angle dependent value. The purpose of the retardation plates 21 and 22 shown in FIG. 1 is to correct the horizontal component which was adversely affected by the liquid crystal material. However, the retarders shown in FIG. 1 have a single retardation value which does not take into consideration the different wavelengths of each color (e.g. red, green, and blue) which have been affected differently by the birefringence of the LC material. In other words, when using a retarder with a constant retardation value, the overall viewing angle of the multi-gap pixel shown in FIG. 1 can be improved, but at different viewing angles, the result is different viewing leakages for each color.
Prior art FIG. 4 illustrates a second type of known NB pixel which includes red, green, and blue subpixels. Normally incident light 101 first passes through a first linear polarizer 103. First linear polarizer 103 has a transmission axis parallel to the transmission axis of second linear polarizer 112, thereby defining a normally black (NB) liquid crystal display pixel. After being polarized by linear polarizer 103, the light 101 then proceeds through a first transparent substrate 104 and transparent subpixel electrodes 105. Each color subpixel has its own electrode 105 which enables a selectively activated voltage to be applied across each subpixel. After passing through electrodes 105, the normally incident light 101 then proceeds into and through a liquid crystal layer 109 having a thickness "d." The liquid crystal layer 109, having a constant thickness throughout the entire pixel, has, at its interface with electrodes 105 a first orientation film (not shown) buffed in a direction substantially perpendicular to the transmission axis of the first polarizer 103. Opposing the first orientation film (not shown) is a second orientation film (not shown) disposed at the interface of the liquid crystal material 109 and color filters 106-108. This second orientation film (not shown) is buffed in a direction substantially parallel to the transmission axes of both the first and second polarizers. The substantially crossed buffing directions of the first and second orientation films (not shown) creates about a 82.degree. -100.degree. twist in the liquid crystal layer 109. In other words, as normally incident light passes through the liquid crystal material 109 from the first orientation film adjacent the electrodes 105 to the second orientation film adjacent the color filters, the light is twisted about 82.degree.-100.degree. . After proceeding through the liquid crystal layer 109, the light then progresses through the aforesaid described second orientation film and the color filters 106-108 of the respective subpixels. The blue subpixel includes a blue color filter 106, the red subpixel a red color filter 107, and the green subpixel a green color filter 108. After passing through one of the color filters, the normally incident light then proceeds through a second transparent substrate 110, a retardation film 111, and a second or exit polarizer 112. The retardation film 111 has a constant retardation value throughout the entire pixel. After passing through the second polarizer 112 which has a transmission axis oriented parallel to the transmission axis of the first polarizer 103, the light proceeds toward a viewer who preferably views the resulting light at an ON axis or normal viewing angle 113. The normal viewing angle N has its axis perpendicular to a plane defined by, for example, the polarizers 103 and 112 of the liquid crystal cell.
The cell gap or thickness "d" of this particular pixel is about 5.70 micrometers and is matched to the first transmission minimum for the color green which has a wavelength of 550 nm. The retardation film 111 has a constant birefringent value (.DELTA.N) which is positive. The optical axis of the retardation film 111 is parallel to the buffing zone of the first orientation film and perpendicular to the transmission axes of the first and second polarizers 103 and 112. The principal drawback, as will be described below, of this prior art pixel shown in FIG. 4 is that the different wavelengths representative of the different colors are not compensated for, the result being a variance in contrast between the colors at different viewing angles.
FIGS. 5-7 are computer simulation graphs illustrating the effect of the pixel of FIG. 4, absent its retardation film, upon red, green, and blue wavelengths respectively.
FIG. 5 is a computer simulation graph illustrating the effect of the pixel of FIG. 4, absent its retardation film 111, on the red light wavelength of 630 nm. The parameters used in simulating this effect shown in FIG. 5, include a cell gap "d" of 5.70 micrometers, a driving ON voltage of 4.0 volts, an OFF voltage of 0.9 volts, and the linear polarizers 103 and 112 having transmission axes parallel to one another and perpendicular to the first buffing zone adjacent the electrodes 105. As can be seen in FIG. 5, the contrast ratio at normal (0.degree. vertical, 0.degree. horizontal viewing angle) is only about 30:1. Furthermore, as one proceeds up and down the 0.degree. horizontal axis (e.g. 0.degree. horizontal, -40.degree. to 40.degree. vertical), the contrast ratio never exceeds about 30:1 and quickly drops below 30:1 at vertical viewing angles of about 7.degree. and -15.degree.. This graph illustrates a twin peak effect meaning that while the contrast ratio is poor at normal, it is improved horizontally on both sides of normal. In other words, the contrast ratio at 0.degree. vertical and 30.degree. horizontal is about 130; and the contrast ratio at 0.degree. vertical and -30.degree. horizontal is about 110:1. As is evident by this graph illustrated in FIG. 5, the red wavelength of 630 nm incident upon the pixel of FIG. 4 absent its retarder, suffers greatly at substantially normal viewing angles, and all vertical viewing angles where the horizontal viewing angle is around 0.degree..
FIG. 6 is a computer simulation of the effect that the pixel of FIG. 4, absent its retardation film, has upon green light with a wavelength of 550 nm. This simulation utilizes as parameters a cell gap of 5.70 micrometers, an ON voltage of 4.0 volts, an OFF voltage of 0.9 volts, and parallel polarizer axes which are perpendicular to the first buffing zone adjacent the electrodes 105. Because the cell gap "d" of the FIG. 4 prior art pixel is matched to the first transmission minimum of the green wavelength of 550 nm used in this simulation, the contrast ratio at normal (0.degree. vertical, 0.degree. horizontal) is very good at about 210:1. The 30:1 contrast ratio curve extends along the 0.degree. horizontal axis from vertical angles of about -27.degree. to about +30.degree. thereby spanning a range along the 0.degree. horizontal axis of about 57.degree.. Furthermore, the 30:1 contrast ratio curve extends along the 0.degree. vertical axis from horizontal angles of about -37.degree. to about +37.degree., thereby defining a horizontal range along the 0.degree. vertical viewing axis of about 74.degree.. The contrast ratio curves of FIG. 6 for the color green are markedly superior to those of FIG. 5 because the cell gap "d" of the FIG. 4 pixel is matched to the first transmission minimum for the color green, while being lower than the first transmission minimum of the color red. Likewise, because the cell gap of the FIG. 4 pixel is matched to the first transmission minimum of the color green and is higher than that needed for the color blue, the contrast ratio graph for the color blue described below with regard to FIG. 7 is inferior to that of the color green shown in FIG. 6.
FIG. 7 is a computer simulation of a graph illustrating the effect of the pixel shown in FIG. 4, absent its retardation film, on the blue wavelength at 480 nm. This graph uses parameters including a cell gap of 5.70 micrometers, an ON voltage of 4.0 volts, an OFF voltage of 0.9 volts, and polarizers having parallel transmission axes perpendicular to the first buffing zone. As can be seen in FIG. 7, because the cell gap "d" of the FIG. 4 pixel is not matched to the first transmission minimum for the color blue, the contrast ratio of the blue wavelength at normal is poor, being less than about 40:1. Furthermore, the 30:1 contrast ratio curve extends along the 0.degree. horizontal axis only to a limitation of about -8.degree. vertical. Also, the same 30:1 contrast ratio curve extends along the vertical 0.degree. axis to horizontal extents of only about -13.degree. and +13.degree.. As will be evident to those skilled in the liquid crystal display art, this is a relatively poor contrast ratio curve indicative of the problems of the prior art FIG. 4 pixel.
FIGS. 8-10 are computer simulation graphs illustrating the contrast ratio curves of the prior art FIG. 4 pixel, including a retardation film having a constant retardation value of 275 nm, with respect to the colors red, green, and blue respectively. These three graphs utilize voltage parameters including a 4.8 V on voltage, and a 0.2 V OFF voltage. The use of a 275 nm retardation film within the FIG. 4 prior art pixel is not considered prior art, but is utilized in these simulation graphs for the purpose of later discussed comparison with certain embodiments of this invention.
FIG. 8 illustrates the contrast ratios given the color red at a wavelength of 630 nm by the prior art pixel shown in FIG. 4 including a 275 nm retardation film. The contrast ratio at normal is only about 30:1. The 30:1 contrast ratio curve extends along the 0.degree. horizontal viewing axis to vertical viewing angles of about -35.degree. and +12.degree.. Again, the contrast ratio curves shown in FIG. 8 for the color red are very poor because the pixel of FIG. 4 including its retardation film having a constant retardation value of 275 nm, does not compensate for the different wavelengths representative of the red, green, and blue colors. Accordingly, because the cell gap of the FIG. 4 pixel is matched to the first transmission minimum of the color green, thereby being below the first transmission minimum for the color red, the resulting contrast ratios for the color red at normal and most other viewing angles are very poor as illustrated in FIG. 8.
FIG. 9 is a computer simulation of contrast ratios for the color green wavelength of 550 nm resulting from the pixel shown in FIG. 4 including a retardation film having a retardation value of 275 nm. Because the cell gap of 5.70 micrometers is matched to the first transmission minimum for the color green, the resultant contrast ratio curves illustrated by FIG. 9 are relatively good. The contrast ratio at normal is about 270:1, while the 30:1 contrast ratio curve extends off the graph along both the vertical and horizontal 0.degree. viewing axes. As discussed previously, the reason for the superior contrast ratios for the color green in the FIG. 4 pixel, is that the cell gap in the pixel is matched to the first transmission minimum for the color green, and furthermore, the retardation film has a retardation value of 275 nm which is also personalized to the color green.
FIG. 10 illustrates a computer simulation graph of contrast ratios for the color blue wavelength of 480 nm propagating through the prior art pixel shown in FIG. 4. As can be seen in FIG. 10, because the cell gap "d" is not matched to the first transmission minimum of the blue wavelength, the contrast ratios are poor. At normal, for example, the contrast ratio is only about 30:1. The 30:1 contrast ratio curve extends horizontally along the 0.degree. vertical viewing axis from about -26.degree. to 26.degree.. Furthermore, the 30:1 contrast ratio curve extends downward along the 0.degree. horizontal axis only to about -9.degree. vertical. Accordingly, it is clear that the prior art pixel shown in FIG. 4 provides poor contrast ratios both horizontally and vertically for the blue wavelength.
It would clearly be a step forward in the art if a liquid crystal display pixel could be provided which displayed good contrast ratios for all colors and eliminated the need for the multi-gap configuration shown in FIG. 1.
U.S. Pat. No. 5,179,457 discloses a liquid crystal display device including a phase plate disposed between a liquid crystal layer and a lower electrode, wherein the phase plate has different amounts of birefringence at different positions thereby creating a color display without using color filter(s). U.S. Pat. No. 5,179,457 does not discuss using such a phase plate in combination with color filters, and is directed toward a different type of LCD than the present invention.
U.S. Pat. No. 5,150,237 discloses an electrically controlled birefringence (ECB) type LCD which utilizes a uniaxial medium having a positive anisotropy arranged between the liquid crystal layer and a polarizer, wherein the products of refractive index anisotropy and thickness of the uniaxial medium are different from each other in accordance with the difference between displayed colors. The ECB display of U.S. Pat. No. 5,150,237 is not directed toward a twisted nematic type LCD which uses color filters as described by the instant invention.
U.S. Pat. No. 5,250,214 discloses a combination of a phase plate and an optical color filter film wherein the phase plate includes a film of liquid crystal polymer composition having polyester as a main constituent.
U.S. Pat. No. 5,229,039 discloses a polyimide based color filter which also functions as an orientation film.
The aforesaid discussed prior art which utilizes both retarders and color filters all utilize one element which functions as a color filter and another separate element which functions as a retarder. It would solve a long-felt need in the art if these two functions could be performed by a single element which functioned as both a color filter and a retardation element.
The term "interior" when used herein to describe a surface or side of an element, means the side or surface closest to the liquid crystal material.
The term "retardation value" as used herein means "d.multidot..DELTA.N" of the retardation film or plate, wherein "d" is the film thickness and ".DELTA.N" is the film birefringence. Defined values may be either positive or negative depending on the birefringence of the film.
The terms "clockwise" and "counterclockwise" mean as viewed from the observer's side of the liquid crystal display.
The term "first" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones, and orientation films means that the described element is on the incident light side of the liquid crystal material, or in other words, on the side of the liquid crystal material opposite the viewer.
The term "second" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones, and orientation films means that the described element is located on the viewer side of the liquid crystal material layer.
The "horizontal viewing angles" (or X.sub.ANG) and "vertical viewing angles" (or Y.sub.ANG) illustrated and described herein (see FIG. 24) may be transformed to conventional LCD angles .phi. and .THETA. by the following equations: EQU TAN (X.sub.ANG)=COS (.phi.).multidot.TAN (.THETA.) EQU SIN (Y.sub.ANG)=SIN (.THETA.).multidot.SIN (.phi.) EQU COS (.THETA.)=COS (Y.sub.ANG).multidot.COS (X.sub.ANG) EQU TAN (.phi.)=TAN (Y.sub.ANG).div.SIN (X.sub.ANG)
FIG. 24 illustrates the relationship between the four different angles.
It is apparent from the above that there exists a need in the art for a multi-colored liquid crystal display pixel wherein the multi-gap need to adjust the cell gap "d" for each color is eliminated and each color wavelength is compensated for, thereby improving the viewing angle and contrast ratios associated therewith for each particular color and substantially eliminating different viewing leakages for different colors at various viewing angles. There also exists a need in the art for a single element which functions as both an optical retarder and a color filter.